Along with the abrupt development of the information technology, technology with which large capacity promotion, increase in port mounting density, and economical efficiency of a system are made compatible with one another has become more and more important not only in the conventional communication network (telecom), but also in the datacom such as a storage area network (SAN) or the Ethernet (registered trademark) (LAN) which has remarkably progressed. The throughput of these high-speed interface systems is limited by the mounting density depending on the module size and the power consumption in addition to the speed per channel port. From this, the miniaturization and power saving promotion of the optical components serving as main components have become a key for determining the total performance of the device.
For this reason, in the semiconductor laser, the semiconductor photo detector and its module which control transmission/reception of an optical signal, how the power consumption is reduced has become an important index.
FIG. 1 is a graphical representation in which transmission light sources used in transmission/reception light source modules each of which operates at 10 gigabits per second are classified with respect to power consumption and a fiber transmission distance. As can be seen from the figure, the fiber transmission distance and the module power consumption show a trade-off relationship. This is because the semiconductor light beams and electronic devices thus used are different from one another with respect to the distance as shown in the figure. In general, a direct modulation system in which a wavelength of a signal light beam is equal to or shorter than about 1.3 μm or about 0.85 μm is used in short distance applications. Making further classification, in ultra-short distance transmission having a distance of several hundreds meters or less, a 0.85-μm-band multimode vertical cavity surface emitting laser (VCSEL) is used as a light source for direct modulation. This multimode vertical cavity surface emitting laser is disclosed in non-patent document 1: Lasers and Electro-optics Society, 2003. LEOS 2003, The 16th Annual Meeting of the IEEE Volume 2, 27-28 Oct. 2003 Page(s): 511-512 vol. 2. On the other hand, in applications for a short distance of about 10 km or less, a 1.3-μm-band single mode edge emitting laser is used as a light source for direct modulation. This 1.3-μm-band single mode edge emitting laser is disclosed in non-patent document 2: Optical Fiber Communications Conference, 2003. OFC 2003, 3-28 Mar. 2003 Page(s): PD40—P 1-3 vol. 3.
In the direct modulation system, the power consumption is little since the module can be realized with a relatively simple construction. In particular, the vertical cavity surface emitting laser operates with a little current of about several milli-amperes to about 10 mA because a micro cavity structure having a sub-micron length is reflected therein. Thus, the vertical cavity surface emitting laser has a laser cavity structure which is very little in power consumption and which has essentially an excellent power saving property. In addition, all the laser cavity structures can be manufactured only in the wafer process, and the inspection and selection process thereof can be carried out in a wafer state. Thus, the vertical cavity surface emitting laser has the excellent feature in terms of the economical efficiency as well.
On the other hand, in order that a 1.3-μm-band edge emitting laser may operate at 10 gigabits per second, it is actually required that the 1.3-μm-band edge emitting laser operates with a current of about 60 mA or more at minimum. For this reason, the power consumption rises extremely up to about double. Thus, for the purpose of applying the vertical cavity surface emitting laser to the applications for a short distance of about 10 km or less, the 1.3-μm-band vertical cavity surface emitting laser is being energetically investigated. However, in the present circumstances, it is difficult for chip light power of a several milliwatt class becoming a key for the practical application to technically realize with a single lateral mode structure. This is because of a luminous layer with a too small volume. As described in non-patent document 3: IEEE 19th International Semiconductor Laser Conference Digest, p. 141, in the case of the 1.3-μm-band vertical cavity surface emitting laser, light power abruptly drops to about several hundreds milliwatts at high temperatures with a luminous area diameter of about 5 μm or less as a simple mode condition. The light power of about 1 mW is obtained when the luminous area diameter is equal to or larger than about 10 μm. However, in this case, the 1.3-μm-band vertical cavity surface emitting laser is forced to operate with a multi-mode.
On the other hand, in the applications for a middle/long distance of about 40 km or more, there is adopted an external modulation system using an optical modulator which operates in a 1.55-μm-band permitting the less transmission loss of the optical fiber. As a result, the power consumption further increases. In addition thereto, in a wavelength division multiplexing (WDM) system transmission, a newly consumed power is added for wavelength stabilization of a wavelength variable light source. Thus, in the present circumstances, an electronic power is compelled to increase several times or more as much as that in the ultra-short distance transmission.
At that, as for known examples each relating to the laser cavity structure of the present invention, there are techniques disclosed in non-patent document 4: IEEE 19th International Semiconductor Laser Conference Digest, p. 115, and in non-patent document 5: IEEE 19th International Semiconductor Laser Conference Digest, p. 143. Each of these techniques relates to an improvement in wavelength variable characteristics of a 1.55-μm-band wavelength variable laser light source. While the operation principles of each of them are different from those of a high-speed direct modulation light source of the present invention, since each of them has a similar device structure, non-patent documents 4 and 5 are described herein with non-patent documents 1, 2 and 3. In addition, as for known examples each relating to the vertical cavity surface emitting laser structure of the present invention, there are techniques disclosed in non-patent document 6: IEEE 18th International Semiconductor Laser Conference Digest, p. 113, and in non-patent document 7: Optical Fiber Communications Conference, 2005, OFC 2005, 6-11 Mar. 2005 OTuM5.